1. Field
The present invention relates to a capacitance-type transducer, an acoustic sensor, and a microphone. Specifically, the present invention relates to a capacitance-type transducer configured by a capacitor structure made up of a vibrating electrode plate (diaphragm) and a fixed electrode plate. Also, the present invention relates to an acoustic sensor (acoustic transducer) that converts acoustic vibration into an electrical signal and outputs the electrical signal, and a microphone that employs this acoustic sensor. In particular, the present invention relates to a small-size capacitance-type transducer and acoustic sensor manufactured using MEMS (Micro Electro Mechanical System) technology.
2. Related Art
FIG. 1 is a cross-sectional diagram showing an example of a conventional capacitance type of acoustic sensor manufactured using MEMS technology. In an acoustic sensor 11 shown here, a cavity 13 is formed in a substrate 12, and a diaphragm 14 (vibrating electrode plate) is provided above the substrate 12 so as to cover the upper opening of the cavity 13. If the diaphragm 14 is rectangular, for example, the four corner portions thereof are supported on the upper surface of the substrate 12 by anchors 15. A dome-shaped back plate 16 is formed on the upper surface of the substrate 12, and the back plate 16 covers the diaphragm 14. A fixed electrode plate 17 is provided on the lower surface of the back plate 16, and the fixed electrode plate 17 opposes the diaphragm 14. The diaphragm 14 and the fixed electrode plate 17 oppose each other, thus configuring a variable capacitor for converting acoustic vibration into an electrical signal. Also, a large number of circular acoustic holes 18 are formed in the back plate 16 and the fixed electrode plate 17. Multiple stoppers 19 are provided on the lower surface of the back plate 16 so as to project from the fixed electrode plate 17. This type of acoustic sensor is disclosed in JP 2011-239324, for example.
The acoustic holes 18 are holes serving as passes for acoustic vibration. For example, acoustic vibration that has entered the acoustic sensor 11 through the acoustic holes 18 causes the diaphragm 14 to vibrate, and then moves to the cavity 13. Alternatively, acoustic vibration that has entered the acoustic sensor 11 through the cavity 13 causes the diaphragm 14 to vibrate, and then moves to the outside through the acoustic holes 18. Also, the stoppers 19 are provided in order to prevent the diaphragm 14 from sticking (adhering) to and not separating from the fixed electrode plate 17. If the stoppers 19 are not provided, there are cases where electrostatic attraction force is generated between the diaphragm 14 and the fixed electrode plate 17 due to the bias voltage applied between the diaphragm 14 and the fixed electrode plate 17, and the diaphragm 14 adheres to and does not separate from the fixed electrode plate 17 due to the electrostatic attraction force. There are also cases where the diaphragm 14 adheres to and does not separate from the fixed electrode plate 17 due to the surface tension of moisture remaining in the air gap between the diaphragm 14 and the fixed electrode plate 17. The stoppers 19 are for preventing the diaphragm 14 from sticking to the fixed electrode plate 17 by coming into contact with the diaphragm 14 before the fixed electrode plate 17 in such cases.
These capacitance type of acoustic sensors may have a reduction in the S/N ratio caused by thermal noise in the air gap. Thermal noise in the air gap is noise generated due to air molecules in the air gap between the diaphragm and the fixed electrode plate (i.e., in a semi-airtight space) colliding with the diaphragm due to fluctuating (thermal motion). The very small force received by the diaphragm due to the colliding air molecules changes randomly due to the fluctuation of the air molecules, and therefore the diaphragm vibrates irregularly due to the colliding air molecules, and electrical noise caused by thermal noise is generated in the acoustic sensor. In high-sensitivity acoustic sensors in particular, this noise caused by thermal noise is large, and the S/N ratio becomes worse.
This noise caused by thermal noise can be reduced by increasing the opening ratio of the acoustic holes formed in the back plate and the fixed electrode plate so as to make it easier for air in the air gap to pass through the acoustic holes. Accordingly, in order to improve the S/N ratio in the above-described acoustic sensor 11 as well, the opening area (opening ratio) of the acoustic holes 18 needs to be increased.
On the other hand, if the diameter (or cross-sectional area) of the stoppers 19 is too small, there are cases where the diaphragm 14 and the stoppers 19 collide due to impact when the acoustic sensor 11 is dropped, and the diaphragm 14 is damaged by the tips of the stoppers 19. The stoppers 19 therefore need to have a certain thickness (or cross-sectional area).
However, if the opening area of the acoustic holes 18 is increased in order to improve the S/N ratio, the area of the region surrounded by the acoustic holes 18 in the back plate 16 decreases, thus making it impossible to provide large-diameter stoppers 19. The diameter of the stoppers 19 therefore needs to be reduced, and the diaphragm 14 is more easily damaged by collision with the stoppers 19 when the acoustic sensor 11 is subjected to drop impact as described above, for example.
Also, since the acoustic holes 18 cannot be provided so as to be overlapped with the positions where the stoppers 19 are located, if the diameter of the stoppers 19 is increased, the opening area of the acoustic holes 18 can no longer be increased, and the S/N ratio of the acoustic sensor 11 decreases.
Accordingly, in conventional acoustic sensors, there is a trade-off relationship between increasing the opening area (opening ratio) of the acoustic holes in order to improve the S/N ratio and increasing the diameter (cross-sectional area) of the stoppers in order to prevent damage to the diaphragm.
Note that with the back plate disclosed in FIG. 3 of U.S. patent application No. 2011/0075866, a large region (region without acoustic holes) that is larger than the other regions is provided in a portion of the back plate. However, there is no recitation of providing stoppers in this wide region in U.S. patent application No. 2011/0075866. Also, the total opening area of the acoustic holes would decrease with such a structure.
JP 2011-239324A and the description of U.S. patent application No. 2011/0075866 (FIG. 3) are examples of background art.